Conventionally, video or audio signals are transmitted according to an analog or a digital modulation system through a ground wave broadcasting path or a cable TV transmission path. Recently a satellite communication and a satellite broadcasting have been developed for transmitting the video signals or the audio signals.
In a system for transmitting video signals or audio signals through a ground wave broadcasting path or a cable TV transmission path, an analog vestigial sideband amplitude modulation (hereinafter referred to as VSB-AM modulation) system is widely used. In particular, the VSB-AM modulation system is exclusively used in the ground wave broadcasting path. This is because the frequency utilization efficiency of the VSB-AM modulation system is excellent in existing technologies available in this field.
The VSB-AM modulation system and a receiver associated with the VSB-AM modulation system will be described in detail, for instance, in a publication titled "Color TV Technology" published from Ohm Inc. in Japan, on Aug. 15, 1979. Principal features of the receiver described in the publication will be summarized below.
1) The VSB-AM modulation system is for the vestigial sideband transmission with a frequency spectrum left around a carrier frequency. Therefore, in order to get flat demodulated output in the entire band width, a spectrum shaping must be performed using a Nyquist filter that has a slope symmetrical to carrier frequency. 2) The demodulated output can be detected through, for example, an envelope detection. However in order to prevent distortions in demodulated signals, a synchronous detection is required.
FIG. 1 shows a block diagram of conventional VSB-AM modulation receiver. After amplified by an RF amplifier 801, a desired channel of an input signal is tuned by a tuner 802 and then the input signal converted into an intermediate frequency band signal (hereinafter referred to as IF signal). The IF signal passes through an automatic gain controller (hereinafter referred to as AGC) 803 and a Nyquist filter 807 and then branched into two paths. One of these branched signals is subject to a carrier regeneration at a carrier regenerator 808 which includes a PLL (phase-locked loop) etc., and then multiplied with the other branched signal at a mixer 809. The demodulation system for detection by the regenerated carrier frequency with a narrow band width regeneration characteristics is a synchronous detection, which has such a merit that no demodulation distortion is produced in principle in compared with an amplitude modulated envelope detection. The demodulated signal is output through the output terminal 811 after removed its harmonic components and noises by a low-pass filter (hereinafter referred to as LPF) 810.
The tuner circuit 802 is capable of obtaining stable local oscillation frequency by using, for instance, a frequency synthesizer. However, since the frequency of the input signal itself is generally not stable, there is a possibility of causing a frequency detuning. For instance, since relatively inexpensive RF modulators as used in many VTRs (video tape recorders) do not necessarily have a stable frequency characteristics, an automatic frequency control (hereinafter referred to as AFC) loop comprising a frequency error (.DELTA.f) detector 804 is provided in tuner circuits of ordinary receivers, as shown in FIG. 1, so as to suppress the frequency detuning in front of the Nyquist filter 807. The frequency error signal .DELTA.f obtained by the .DELTA.f detector 804 is supplied to a frequency control terminal of a local oscillator 805.
In recent years, a digital ground wave broadcasting system and a digital cable transmission system were developed for transmitting digital video or audio signals through a path having the same transmission band width as that of conventional analog transmission systems, through direct digital modulation of signals after encoding them at a high efficiency coding. Such systems have a feature that they are capable of transmitting much more information than the conventional analog modulation system as described above and are free from an affect of transmission noises or the like, and are thus expected that they will be extensively used in the future.
As a digital modulation system used for the digital broadcasting, a quadrature amplitude modulation (hereinafter referred to as QAM) which is widely used in the communication field is considered to be the most general modulation system. With a multivalued QAM, such modulations as 16QAM, 32QAM, 64QAM and 256QAM are proposed and are put to practical use.
Demodulators of the modulation system are conventionally put into practical use mostly in digital microwave circuits and are seen, for instance, in a paper titled "Construction and Characteristics of 16QAM Carrier Regenerator Retaining Mode Switching Function" by MATSUE et al., in the "Electronic Communication Society Paper", 1985/3, vol. J68-B, No. 3.
FIG. 2 shows the conventional general multivalued QAM demodulator. The QAM signal input to an input terminal 830 is amplified and tuned by an RF (high frequency) amplifier 831, a tuner 832, a local oscillator 833 and an AGC amplifier 834 similarly to those illustrated in FIG. 1, and then converted to an IF signal. The IF signal is supplied to a quadrature detector 835 wherein the IF signal is branched into an in-phase detector 836 and a quadrature-phase detector 837. A local oscillation signal generated from a local oscillator 838 is supplied to the in-phase detector 836 as a 0.degree. phase local oscillation signal and is supplied to the quadrature-phase detector 837 as a 90.degree. phase local oscillation signal via a 90.degree. phase shifter 839. The outputs from the detectors 837 and 836 are supplied to low-pass filters (LPFs) 841 and 842, respectively, which have the same frequency transfer characteristics for spectrum shaping. The LPFs 841 and 842 have the transfer characteristics demanded for preventing an inter-symbol interference in the digital data transmission path and are designed to form a so-called roll-off characteristics over a transmitting section and a receiving section in many cases. Therefore, in the outputs of the LPFs 841 and 842, an eye opening rate is made sufficiently large and are converted to digital values at the timing of the center of the eye opening in A/D converters 843 and 844. As the digitized demodulated outputs have a symbolic amplitude on the in-phase axis and a quadrature-phase axis, it is possible for a data identifier 845 to identify each symbol from the digitized demodulated output.
Further, the eye opening centered timing clock is produced in a clock regenerator 846. In addition, in the data identifier 845, the phase of transmitted symbols is detected from amplitudes of symbols on the in-phase axis and the quadrature-phase axis and then fed back to the local oscillator 838 as a phase error signal of a carrier regenerator, a type of PLL circuits, via a loop filter 847 and a D/A converter 848, thus achieving a phase synchronization between the input signal and the regenerated carrier signal.
Now, assuming a receiver that is commonly usable to an analog modulated signal and a digital modulated signal, as shown in FIGS. 1 and 2, a receiver having another configuration, as shown in FIG. 3, can be conceived. The elements up to the element for generating the IF signal they are identical to those in the above configuration (e.g., FIG. 1) so that explanations therefor will be omitted hereinafter. The IF signal is supplied to both an analog modulated signal demodulator 851 and a digital modulated signal demodulator 852. Then, after processed pertinently in the demodulators 851 and 852, the outputs of these demodulators 851 and 852 are supplied to a signal processor 853 and a digital decoder 854. The outputs from the signal processor 853 and the digital decoder 854 are supplied to a switch 855, thus a pertinent signal is selected by the switch 855 and then led to an output terminal 856. The switch 855 changes over when a change-over signal is provided to the switch 855 by an operator, from an operation unit via a terminal 857.
Conventionally, for instance, a VSB-AM modulation, a type of analog modulations, and a QAM modulation, a type of digital modulations, have been used in separate transmission systems, respectively. So that there was no transmission or broadcasting system in which these two types of modulations are mixed together in use. However, as a result of the recent development of the digital transmission technology as described above, it has become possible to use the QAM modulation, even in a transmission system or a broadcasting system in which the VSB-AM modulation has been conventionally used. As a result, with the widespread use of the digital transmission systems, the state in which both modulation systems are used is expected and receivers adaptive to both modulation systems will be demanded in the future.
In the arrangement as described above, however, each of the analog and digital demodulators is independent and thus users must know in advance a modulation system by which a signal is transmitted or broadcasted, and then select an adequate demodulator. However, the type of the modulation system is not interested for users or it will obstruct the widespread use of receivers to force users the selection between the modulation systems.
If it is attempted to incorporate an analog demodulator and a digital demodulator together, different demodulators must be provided. And if these modulation systems are used for the transmission or the broadcasting system together, two types of demodulators must be provided. That is, the receiver antenna, the RF amplifier, frequency converter, etc. can be commonly used for both the modulation systems, but either type of demodulator cannot be used for both the modulation systems. Therefore, there is a problem of reducing costs of the receivers.
On the other hand, an analog frequency modulation (hereinafter simply referred to an FM modulation) is mainly used in a satellite transmission system of video signals or audio signals. In particular, in a satellite broadcasting system the FM modulation is generally used as its modulation system and is also used as broadcasting systems at many countries as well as Japan. This is because the FM modulation is excellent in the S/N properties after demodulated when compared with amplitude modulation, etc.
An example of conventional FM demodulators is described in detail in a book titled "Comprehensible FM Technology" by Yuya ITOH and Akira FUJII, published by Sanpo Shuppan Inc., in 1968. Typical components of the FM demodulator include a double tuner and a PLL (phase synchronization loop) which is adopted by many FM demodulators available on the market at present.
FIG. 4 shows an example of the FM demodulator employing such a conventional PLL circuit. In the diagram, the PLL circuit is illustrated in a simple fashion in order to make the explanation easy. In the diagram, an FM signal converted to an IF signal at a frequency converter 901 in order to facilitate detection is supplied to a phase detector 902. The phase detector 902 multiplies the output of a voltage control oscillator 903, which will be described later, by the input FM signal and outputs the result of the multiplication to a loop filter 904. The loop filter 904 removes higher harmonic components of the phase detection output, and also removes undesired noise component. The output from the loop filter 904 is supplied to the frequency control input terminal of the voltage control oscillator 903 for resulting a configuration of a PLL. In the state where the PLL is operating normally, the output of the loop filter 904, that is, the input to the voltage control oscillator 903 changes following instantaneous frequency of input FM signal. Therefore, if the signal is taken out, an FM demodulated output is obtained.
Further, to compensate a frequency detuning of the transmission system which generally exists, an automatic frequency control (AFC) is used. If the FM signal is being transmitted in such a manner that its average frequency becomes specified, the AFC in the FM modulation obtains a average frequency of the FM signal at the receiving side and if the average frequency is out of a specified value, the AFC controls local oscillation frequency of a frequency converter (for instance, the frequency converter 901) so that its output will have the specified average frequency.
In recent years, a satellite communication or a satellite broadcasting which is to digitally transmit digital video or audio signals by digitally modulating signals directly is proposed. This system has such features that the signals are free from transmission noises when compared with the FM modulation and is expected to widely spread in the future.
As the digital modulation to be used for the digital satellite broadcasting, a quadrature phase modulation (or quadrature phase shift keying (hereinafter referred to as QPSK modulation), which is extensively used in the field of communication, is expected to become a typical modulation system. Many demodulators in the modulation system are conventionally put to practical use as seen in, for instance, a paper titled "Development of Digital Demodulation LSI for Satellite Communication" by Yagi and others, that is disclosed in the Autumn National Meeting of electronic Information Communication Society (1990). FIG. 5 shows a conventional QPSK demodulator made on an integrated circuit for improving its performance by using a digital circuit.
The QPSK modulated signal received on an input terminal 921 is branched into an in-phase detector 922 and a quadrature-phase detector 923. Local oscillation signals supplied to the detectors 922 and 923 are 0.degree. and 90.degree. phase local oscillation signals branched from a fixed frequency output of a local oscillator 925 by a distributer 924. The detection outputs from the detectors 922 and 923 are supplied to analog-digital (A/D) converters 926 and 927 and then converted into digital values, respectively. Further, the digitized detection outputs are supplied to a complex multiplier 928. On the other hand, sine and cosine characteristics signals are supplied to the complex multiplier 928 from a sine converter 938 and a cosine converter 939 which will be described later, and wherein a complex multiplication is carried out. The complex multiplier 928 is capable of realizing completely the same operation as those in a frequency converter in the intermediate frequency band, i.e., a mixer. The result of complex multiplication is supplied to digital low-pass filters 931 and 932 having the same frequency characteristics for spectrum shaping. These digital low-pass filters 931 and 932 have transmission characteristics demanded for preventing an inter-symbol interference in the digital data transmission and are generally so designed that a so-called roll-off characteristics are obtained when combined with filter characteristics in a transmitter side. Therefore, the detection outputs of the digital low-pass filters 931 and 932 are spectrum shaped so that the eye opening rate is made sufficiently large.
The outputs from the digital low-pass filters 931 and 932 are branched and one of the branched outputs in supplied to a clock regenerator 933 while another output is supplied to a phase detector 934, where a phase error from a reference phase is detected. The phase error detection output (phase error information) from the detector 934 is supplied to a data discriminator 935. The data discriminator 935 discriminates QPSK data from phase information, that is, demodulates and outputs therefrom the QPSK data.
The phase error information from the phase detector 934 is supplied to the frequency control terminal of a numerical control oscillator (NCO) 937 via a loop filter 936 for regenerating the carrier frequency for a synchronous detection. The numerical control oscillator 937 includes a cumulative adder which does not inhibit an overflow and performs the adding operation up to a dynamic range corresponding to a signal value supplied to the frequency control terminal. Therefore, the numerical control oscillator 937 is put into an oscillating state wherein the oscillation frequency is changed by a numerical value of a control signal. The output of the numerical control oscillator 937 is branched into two parts and then supplied to a sine converter 938 and a cosine converter 939, respectively. The outputs of these converters 938 and 939 are supplied to the complex multiplier 928. The loop thus formed hereinabove construct a PLL in a complete digital fashion.
The demodulator described above in not considered a counterplan against a frequency detuning occurring in the transmission system, and therefore an AFC loop additionally required for preventing such a frequency detuning.
The FM modulation, a type of analog modulations, and the QPSK modulation, a type of digital modulations, are conventionally used in independent transmission systems, respectively. And there was no transmission or broadcasting system in which both modulation systems are mixed together in use. However, with the recent development of the digital transmission technology as described above, it has become possible to use the QPSK modulation in the transmission and broadcasting systems conventionally employing the FM modulation. So, it is expected that the state where both modulation systems are mixed together in use will come in the future.
However, even when both the modulation systems exist in the mixed state, two problems will occur as discussed below.
1) When the modulation systems are independent from each other, users must be aware of which modulation system is used for the transmission or the broadcasting, and then select an adequate modulator. For users, what kind of modulation system is used is not of interest and it will impede widespreading use to force users to make the selection of modulator.
2) Since both the FM demodulator and the QPSK demodulator are independent from each other as described above, therefore if these demodulators are used in the mixed state in the transmission or the broadcasting, it is necessary to provide both the demodulators for use in combination.
If the VSB-AM signal, a type of analog modulated signals, and the QAM signal, a type of digital modulated signals, are present in the mixed state in a ground wave broadcasting path or a cable TV transmission path as described above, users must be aware of what kind of modulation system is used for signals being received and it becomes necessary to change over demodulators and this will make it troublesome to receive signals adequately.
In addition, if a plurality of independent demodulators are provided for adaptively receiving signals of respective modulation systems, the entire receiver will become large and expensive.
Further if the FM signal and the QPSK modulated signal are mixed together in use in a satellite communication and a satellite broadcasting, at least two modulators must be provided in correspondence with both modulation systems. In such a case, users must be aware of what kind of modulation system is used for signals being received in advance and it becomes also necessary to change over demodulators for use and this will make it inconvenient to use receivers.
Furthermore, if receivers which are capable of receiving different modulated signals are simply combined, the assembly will become very expensive.